This invention relates to the direct current electrobrightening of aluminium and aluminium-base alloys in alkaline electrolytes.
In our British Pat. Nos. 449162 and 513530 there is described a process for electrobrightening aluminium which has become widely known commercially by the Registered Trade Mark "Brytal" and employing an electrolyte including an aqueous solution of Na.sub.2 CO.sub.3 and Na.sub.3 PO.sub.4 with certain optional additives, such electrolyte having a pH of at least 10 and a preferred operating temperature of 75.degree.-85.degree. C. It is stated therein that with this process the specular reflectivity of commercially pure sheet can be "raised perhaps to 80%". More specifically, the specular reflectivity values measured subsequently by the co-applicant, Pullen, and his co-workers, on metal of 99.7% and 99.5% purity, were 79% and 72% respectively. These values for specular reflectivity, together with those given by Pullen and Scott (Trans. inst. of Met. Finishing 1956, vol 33, pp. 163-176) were all obtained with the Guild Photometer in which the semi-angle subtended by the diaphragm aperture employed is 141/2.degree., thereby introducing a significant component of diffuse reflectivity into the measurements and so giving artificially high results.
To overcome this defect, a Modified Gloss Head, employing a diaphragm aperture subtending a semi-angle of about 1.degree. was developed by Scott and this has been adopted as a standard method for specular reflectivity measurements in British Standard 1616: 1972, where it is described in Appendix Q. Data given by Scott and Bigford (A.D.A. Conference on Anodising, Nottingham, 1961, Session 11 paper 4) illustrate the effect of replectometer acceptance angle on measured specular reflectivity; thus, in a typical case, a surface with a specular reflectivity of only 63% when measured with the Modified Gloss Head, accepting a semi angle of 1.degree., showed a value well in excess of 80% when measured with a 141/2.degree. semi angle of acceptance. All references hereinafter to specular reflectivity will relate to the more accurate measurements made in accordance with the aforementioned British Standard specification.
It is well established as the result of many years of commercial operation that the Brytal process gives excellent results on metal of 99.99% purity and on certain alloys made therefrom, the specular reflectivity values obtained reaching or even exceeding 85%. However, with decreasing metal purity the specular reflectivity falls off rapidly, being at best about 75% for 99.8% and 60% for 99.5% base metal purity. In recent years general commercial practice has been to make additions of NaOH to the Brytal bath, thereby raising the pH above the original preferred value of about 11.5 (as measured in the hot solution by glass electrode) increasing current density thereby and gaining in speed of treatment, but without any beneificial effect on brightness.
Because of the mediocre results obtained with metal of base purity less than 99.99% many producers of bright anodised aluminium components have preferred to use chemical brightening processes involving dipping the component in a hot acid bath, typically comprising a mixture of phosphoric and nitric acids, without application of electric current. With one such process, specular reflectivity values of at least 80% (ie an increase of up to 30% over the "Brytal" process) are obtainable even on metal of only 99.5% base purity. Processes of this type are however unpleasant to operate, and there has been a recognised need for many years for an environmentally satisfactory chemical or electrochemical brightening process that could deal effectively with metal of less than 99.99% base purity.
Other types of alkaline electrolyte have been proposed for use in electropolishing aluminium, for example (a) caustic alkali with an aluminium complexing agent, such as sodium gluconate as disclosed in British Pat. No. 1,070,644, (b) alkali metal cyanide alone or with an alkali metal thiocyanate as disclosed in British Pat. No. 655,514, (c) alkali metal hydroxide and orthophospate (without carbonate), as disclosed in British Pat. No. 658,699 and (d) electrolytes containing free caustic soda as disclosed in British Pat. Nos. 521,290 and 1,070,644, but only electrolytes of the sodium carbonate-trisodium phosphate (i.e. Brytal) type found wide-spread commercial application. Many possible additives to Brytal type baths, including ammonia, ammonium salts, substituted ammonias, bicarbonates and acid phosphates, were mentioned in the original patents or have since been tried over the years, but although in some instances improvements in specular reflecticity may have been obtained the results on low purity alloys have never matched the high quality of surface finish obtainable on the same alloys treated by the chemical brighteners of phosphoric acid - nitric acid type.
In reconsidering the "Brytal" process of our previous early British Pats. 449,162 and 513,530 and the many variations which have been made to it throughout the world during over forty years of commercial usage we have taken into consideration the changes in standards of pH measurement over the same period.
Using the most accurate techniques now available the pH of solutions of two grades, pure and technical, of both sodium carbonate and tri-sodium orthophosphate and the four possible "Brytal" electrolytes made up from these was measured at temperatures of 20.degree.-90.degree. C. Full results are given in Table 1.
TABLE 1 ______________________________________ pH of Carbonate and phosphate solutions and original type Brytal electrolytes Solution of Conc. pH Chemicals g/l 20.degree. 40.degree. 60.degree. 70.degree. 80.degree. 90.degree. ______________________________________ Na.sub.2 CO.sub.3 (analytical grade) 150 11.9 11.5 11.2 11.1 10.9 10.85 Na.sub.2 CO.sub.3 (technical grade) 150 10.95 10.75 10.6 10.6 10.45 10.4 Na.sub.2 CO.sub.3 Na.sub.3 PO.sub.4. 12H.sub.2 O, 50 12.85 12.25 11.8 11.7 11.45 11.25 pure Na.sub.3 PO.sub.4 Na.sub.3 PO.sub.4 Dried, 50 12.45 11.95 11.65 11.53 11.30 11.20 Technical grade Na.sub.3 PO.sub.4 Brytal Electrolytes Na.sub.2 CO.sub.3 (analytical grade) 150 12.9 12.3 11.9 11.7 11.4 11.35 +Na.sub.3 PO.sub.4. 12H.sub.2 O 50 (pure) Na.sub.2 CO.sub.3 (technical 150 11.70 11.45 11.30 11.20 11.05 10.95 grade) +Na.sub.3 PO.sub.4. 50 Dried (Technical grade) Na.sub.2 CO.sub.3 (analytical 150 12.20 11.85 11.6 11.45 11.25 11.15 grade) +Na.sub.3 PO.sub.4. 50 Dried (Technical grade) Na.sub.2 CO.sub.3 (technical 150 12.20 11.8 11.6 11.45 11.2 11.1 grade) +Na.sub.3 PO.sub.4. 12H.sub.2 O 50 (pure) ______________________________________
At 20.degree. C., pure grades of both carbonate and phosphate give solutions, at Brytal concentration, of substantially higher pH than technical grades, i.e. 0.95 and 0.4 pH units respectively.
The 150 g/l/50 g/l Brytal electrolytes also differ considerably, the all-pure grade electrolyte having a pH 1.2 units higher than the all-technical grade electrolyte, while the two electrolytes made up from one pure and one technical grade component give identical pH values at roughly midway between all-pure and all-technical electrolytes.
At working temperature, of 70.degree.-90.degree. C., pH values of the solutions of simple substances and of electrolytes fall by about 0.5-1.0 units pH while pH differences between pure and technical grades is roughly halved.
It will be noted that none of the electrolytes has a pH less than 11.70 at 20.degree. C. and 10.95 at 90.degree. C.
It is therefore concluded that the pH of the original Brytal electrolyte as measured by our glass electrode procedure at 20.degree. Was in fact in the range 11.7-12.9, while at the working temperature specified, i.e. 75.degree.-85.degree., it was in the range 11.0-11.6 according to purity of chemicals, pure chemicals giving the higher value.
Thus we believe that had current pH measuring techniques been available at the time of filing the applications resulting in British Pat. Nos. 449,162 and 513,530 references to a pH value as low as 10 would not have been made and we now know that the "Brytal" process as disclosed in these specifications will not work satisfactorily unless the electrolyte is maintained at a higher pH value. This belief is confirmed by L. Laser in Galvanotechnik 1971 62(a) 779-784 where it is stated "The pH value of the electrolyte is about 10.5 to 12." although even here we think the lowest level referred to is misleading.
It is an object of the present invention to provide an alkaline electropolishing process capable of giving good results on both low purity and high purity base metal.